Current optical test equipment for display testing is either a single-point high accuracy system or a low accuracy imaging system covering a large spatial area. These two types of systems target on different testing objectives. The single-point system will test the display parametric data, including the brightness, color and derived properties, such as contrast, uniformity, gamma and gamut. But the testing only focuses on a single point and not a large spatial area. The imaging system, on the other hand, will test for display artifacts by covering a large spatial area, but it will not have the high accuracy of a single-point system.
A single-point high accuracy system is also known as a narrowband instrument. Spectrometer (or spectrophotometers) and spectroradiometers are examples of narrowband instruments. These instruments typically record spectral reflectance and radiance respectively within the visible spectrum in increments ranging from 1 to 10 nm, resulting in 30-200 channels. They also have the ability to internally calculate and report tristimulus coordinates from the narrowband spectral data. Spectroradiometers can measure both emissive and reflective stimuli, while spectrometers can measure only reflective stimuli. A low accuracy imaging system covering a large spatial area is also known as a broadband instrument. A broadband measurement instrument reports up to 3 color signals obtained by optically processing the input light through broadband filters. Photometers are the simplest example, providing a measurement only of the luminance of a stimulus. Their primary use is in determining the nonlinear calibration function of displays. Densitometers are an example of broadband instruments that measure optical density of light filtered through red, green and blue filters. Colorimeters are another example of broadband instruments that directly report tristimulus (XYZ) values, and their derivatives such as CIELAB (i.e., CIE 1976 (L*, a*, b*) color space). Under the narrowband category fall instruments that report spectral data of dimensionality significantly larger than three.
The main advantage of broadband instruments such as densitometers and colorimeters is that they are relatively inexpensive and can read out data at very fast rates. However, the resulting measurement is only an approximation of the true tristimulus signal, and the quality of this approximation varies widely depending on the nature of the stimulus being measured. Accurate colorimetric measurement of arbitrary stimuli under arbitrary illumination and viewing conditions requires spectral measurements afforded by the more expensive narrowband instruments. Compared with measuring instruments without spatial resolutions, such as spectrometers, this technology offers the following advantages: (a) Substantial time-savings with simultaneous capture of a large number of measurements in a single image, and (b) Image-processing functions integrated in the software allow automated methods of analysis, e.g. calculation of homogeneity or contrast. However, the absolute measuring precision of imaging photometers and colorimeters is not as high as spectrometers. This is because of the operational principle using a CCD (charge-coupled device) sensor in combination with optical filters, which can only be adapted to the sensitivity of the human eye with limited precision. Therefore, the imaging colorimeters are the instruments of choice for measurement of luminance and color distribution of panel graphics and control elements in the display test industry, including but not limited to homogeneity, contrast, mura (i.e., luminance non-uniformity of a display device) and MTF (Modulation Transfer Function).
Therefore, what is desired is an optical test equipment/method for display testing that can perform the functionalities of both the single-point high accuracy system and the low accuracy imaging system at the same time (i.e., parallel testing/sensing configuration that covers spectrum and colorimetric quantities with spatial resolution).